1. Field of the Invention
This invention relates generally to improving voice or data communication channel quality and, more specifically, to adaptive echo cancelers.
2. Description of the Related Art
Communication channel echo is a reflective signal that can impair voice and data communication. Echo occurs when an undesirably reflected communication signal returns to the source after a delay. The impairment can range from mild to severe, depending upon signal energy and delay. At worst case, an echo signal may have enough energy or delay to render conversation unintelligible in analog voice communication networks and corrupt data in digital communication networks. Communication networks commonly include circuits known as hybrids that interface 2-wire network links to 4-wire network links. Typically, 2-wire links are used for voice communication, and 4-wire links are used for data communication. Hybrids are the primary sources of signal reflection in telecommunications networks.
Echo suppressors are circuits that operate in half-duplex communication channels by attenuating the return path signal. A major disadvantage of echo suppressors is that they operate only in half-duplex channels. That is, they cannot suppress echoes where two sources, such as parties to a conversation, communicate with each other simultaneously on the same channel. Echo suppressors therefore have been largely supplanted by echo cancelers. Echo cancellation overcomes the problems of echo suppression by employing adaptive filter algorithms that perform system identification with the communication impulse response over time. In essence, an echo canceler models the hybrid or other reflective medium and subtracts a replica of the echo from the return signal. The adaptive filter algorithm that models the reflective medium adjusts filter coefficients until the filter converges, i.e., accurately models the medium, in response to feedback. The convergence process is commonly referred to as training the filter.
Spurious signals in the communication channel may hamper the operation of echo cancelers. A telephone dialtone is an example of such a signal. A dialtone is composed of two sinusoids, one having a frequency of 350 Hz and the other having a frequency of 440 Hz. Most echo cancelers cannot operate properly when a dialtone is present. It has been suggested that if an echo canceler were forced to train in the presence of high-amplitude interfering sinusoidal signals, it should operate with a greatly reduced adaptation step size in order to reduce the effect of the non-correlated interfering signals upon the accuracy of the filter coefficients. Operating in this manner would slow filter convergence. The higher the amplitude of the interfering signal relative to the correlated echo, i.e., the training signal, the slower the filter converges for a given accuracy.
It would be desirable to provide an adaptive echo canceler that can be trained in the presence of sinusoidal signals such as a dialtone without sacrificing performance. The present invention addresses these problems in the manner described below.
The present invention relates to a system and method for canceling both an echo signal and a spurious sinusoidal signal, such as a telephone dialtone or a carrier signal, from a return signal in a communications system. A replica of the echo signal is produced using an adaptive filter. The replica is subtracted from the return signal. The magnitude and phase of the sinusoidal signal are determined by generating a signal that is close in frequency to the expected frequency of the sinusoid and then correlating it to the return signal. A replica of the sinusoid is produced in response to the correlation. This replica is subtracted from the return signal.
Although the terms echo canceler and echo signal are used for purposes of convenience and clarity herein, the system and method relate to canceling interfering signals other than echoes along with the interfering sinusoid.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.